Abstract:

Bit line-to-bit line noise is discharged in a NAND string prior to sensing
a programming condition of a selected non-volatile storage element in the
NAND string. A source voltage is applied which boosts the voltage in
conductive NAND strings. The voltage boost results in capacitive coupling
of noise to neighboring NAND strings. A current pull down device is used
to discharge each NAND string prior to performing sensing. After each
NAND string is coupled to a discharge path for a predetermined amount of
time, bit lines of the NAND string are coupled to voltage sense
components for sensing the programming condition of the selected
non-volatile storage elements based on a potential of the bit lines. The
selected non-volatile storage elements may have a negative threshold
voltage. Further, a word line associated with the selected non-volatile
storage elements may be set at ground.

Claims:

1. A method for operating a non-volatile storage system, comprising:during
a first time period: (a) applying a source voltage to a source of each of
a plurality of NAND strings, each of the plurality of NAND strings is
associated with a respective bit line, (b) preventing coupling of each
respective bit line to a respective sense component, and (c) coupling
each bit line to a respective discharge path; andduring a second time
period which follows the first time period, continuing to apply the
source voltage to the source of each of the plurality of NAND strings,
and allowing coupling of each respective bit line to the respective sense
component.

3. The method of claim 1, wherein:a respective transistor is coupled
between each respective bit line and the respective sense component, and
the coupling of step (c) comprises providing control gate voltages to the
respective transistors to provide the respective transistors in a
conductive state.

4. The method of claim 1, further comprising:during the second time
period, continuing to couple each bit line to its respective discharge
path.

5. The method of claim 1, further comprising:regulating discharging of
each bit line via the respective discharge path.

6. The method of claim 1, further comprising:regulating discharging of
each bit line via the respective discharge path using a current mirror.

7. The method of claim 1, wherein:the first time period has a
predetermined duration which is selected to allow bit line noise due to
capacitive coupling from one or more neighboring bit lines to be
discharged.

8. The method of claim 7, wherein:the bit line noise is coupled to a bit
line for one of the NAND strings for which an associated selected
non-volatile storage element is in a non-conductive state, by one or more
neighboring bit lines for which one or more associated selected
non-volatile storage elements are in a conductive state.

9. A method for operating a non-volatile storage system,
comprising:applying a source voltage to a source of each of a plurality
of NAND strings, each of the plurality of NAND strings is associated with
a respective bit line;coupling each bit line to a respective discharge
path; andafter the coupling, determining a programming condition of a
selected non-volatile storage element in each of the plurality of NAND
strings by determining whether charge sharing occurs between the
respective bit line and a respective voltage sensing component.

10. The method of claim 9, further comprising:the determining of the
programming condition comprises determining whether the selected
non-volatile storage elements are in a conductive or non-conductive
state.

11. The method of claim 9, further comprising:applying a control gate
voltage to control gates of the selected non-volatile storage elements,
the source voltage and the control gate voltage are set to determine the
programming condition of the selected non-volatile storage elements.

12. The method of claim 11, wherein:the programming condition is
determined with respect to a negative threshold voltage, and the control
gate voltage is a ground voltage.

13. The method of claim 12, further comprising:applying a ground voltage
to a p-well of a substrate on which the plurality of NAND strings are
formed during the determining of the programming condition.

14. A method for operating a non-volatile storage system,
comprising:applying a source voltage to a source of a first set of
storage elements, the first set of storage elements is associated with a
first bit line;coupling a second bit line which is associated with a
second set of storage elements to a respective discharge path to
discharge, at least in part, a potential which is capacitively coupled to
the second set of storage elements from the first set of storage elements
when the source voltage is applied; anddetermining a programming
condition of a selected non-volatile storage element in the second set of
storage elements after the potential is at least partly discharged.

15. The method of claim 14, wherein:the first and second sets of storage
elements are in respective adjacent NAND strings.

16. The method of claim 14, wherein:the first and second sets of storage
elements are adjacent to one another.

17. The method of claim 14, further comprising:applying the source voltage
to a source of the second set of storage elements concurrently with the
applying of the source voltage to the source of the first set of storage
elements.

18. The method of claim 14, further comprising:adjusting a voltage to a
control gate of a transistor which is between a sense component and the
second bit line for the determining of the programming condition.

19. The method of claim 14, further comprising:applying a source voltage
to a source of a third set of storage elements, the third set of storage
elements is associated with a third bit line, and the second bit line is
coupled to it respective discharge path to discharge, at least in part, a
potential which is capacitively coupled to the second set of storage
elements from the third set of storage elements when the source voltage
is applied.

20. The method of claim 14, further comprising:coupling the second bit
line to a sense component for the determining of the programming
condition.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001]This application is related to the following co-pending, commonly
assigned U.S. patent applications:

[0005]Semiconductor memory has become increasingly popular for use in
various electronic devices. For example, non-volatile semiconductor
memory is used in cellular telephones, digital cameras, personal digital
assistants, mobile computing devices, non-mobile computing devices and
other devices. Electrically Erasable Programmable Read Only Memory
(EEPROM) and flash memory are among the most popular non-volatile
semiconductor memories. With flash memory, also a type of EEPROM, the
contents of the whole memory array, or of a portion of the memory, can be
erased in one step, in contrast to the traditional, full-featured EEPROM.

[0006]Both the traditional EEPROM and the flash memory utilize a floating
gate that is positioned above and insulated from a channel region in a
semiconductor substrate. The floating gate is positioned between the
source and drain regions. A control gate is provided over and insulated
from the floating gate. The threshold voltage (VTH) of the
transistor thus formed is controlled by the amount of charge that is
retained on the floating gate. That is, the minimum amount of voltage
that must be applied to the control gate before the transistor is turned
on to permit conduction between its source and drain is controlled by the
level of charge on the floating gate.

[0007]Some EEPROM and flash memory devices have a floating gate that is
used to store two ranges of charges and, therefore, the memory element
can be programmed/erased between two states, e.g., an erased state and a
programmed state. Such a flash memory device is sometimes referred to as
a binary flash memory device because each memory element can store one
bit of data.

[0008]A multi-state (also called multi-level) flash memory device is
implemented by identifying multiple distinct allowed/valid programmed
threshold voltage ranges. Each distinct threshold voltage range
corresponds to a predetermined value for the set of data bits encoded in
the memory device. For example, each memory element can store two bits of
data when the element can be placed in one of four discrete charge bands
corresponding to four distinct threshold voltage ranges.

[0009]Typically, a program voltage VPGM applied to the control gate
during a program operation is applied as a series of pulses that increase
in magnitude over time. In one possible approach, the magnitude of the
pulses is increased with each successive pulse by a predetermined step
size, e.g., 0.2-0.4 V. VPGM can be applied to the control gates of
flash memory elements. In the periods between the program pulses, verify
operations are carried out. That is, the programming level of each
element of a group of elements being programmed in parallel is read
between successive programming pulses to determine whether it is equal to
or greater than a verify level to which the element is being programmed.
For arrays of multi-state flash memory elements, a verification step may
be performed for each state of an element to determine whether the
element has reached its data-associated verify level. For example, a
multi-state memory element capable of storing data in four states may
need to perform verify operations for three compare points.

[0010]Moreover, when programming an EEPROM or flash memory device, such as
a NAND flash memory device in a NAND string, typically VPGM is
applied to the control gate and the bit line is grounded, causing
electrons from the channel of a cell or memory element, e.g., storage
element, to be injected into the floating gate. When electrons accumulate
in the floating gate, the floating gate becomes negatively charged and
the threshold voltage of the memory element is raised so that the memory
element is considered to be in a programmed state. More information about
such programming can be found in U.S. Pat. No. 6,859,397, titled "Source
Side Self Boosting Technique For Non-Volatile Memory," and in U.S. Patent
App. Pub. 2005/0024939, titled "Detecting Over Programmed Memory,"
published Feb. 3, 2005; both of which are incorporated herein by
reference in their entirety.

SUMMARY OF THE INVENTION

[0011]The present invention provides a method for sensing a programming
condition of non-volatile storage elements using all bit line sensing.

[0012]In one embodiment, a method for operating a non-volatile storage
system includes, during a first time period: (a) applying a source
voltage to a source of each of a number of NAND strings, each of the NAND
strings is associated with a respective bit line, (b) preventing coupling
of each respective bit line to a respective sense component, and (c)
coupling each bit line to a respective discharge path. The method further
includes, during a second time period which follows the first time
period, continuing to apply the source voltage to the source of each of
the NAND strings, and allowing coupling of each respective bit line to
the respective sense component.

[0013]In another embodiment, a method for operating a non-volatile storage
system includes applying a source voltage to a source of each of a number
of NAND strings, where each of the NAND strings is associated with a
respective bit line, coupling each bit line to a respective discharge
path, and, after the coupling, determining a programming condition of a
selected non-volatile storage element in each of the NAND strings
according to a potential of each respective bit line.

[0014]In another embodiment, a method for operating a non-volatile storage
system includes applying a source voltage to a source of a first set of
storage elements, where the first set of storage elements is associated
with a first bit line, coupling a second bit line which is associated
with a second set of storage elements to a respective discharge path to
discharge, at least in part, a potential which is capacitively coupled to
the second set of storage elements from the first set of storage
elements, and determining a programming condition of a selected
non-volatile storage element in the second set of storage elements after
the potential is at least partly discharged.

[0039]FIG. 10b depicts an example of an array of storage elements,
including different sets of NAND strings.

[0040]FIG. 11 is a block diagram of a non-volatile memory system using
single row/column decoders and read/write circuits.

[0041]FIG. 12 is a block diagram of a non-volatile memory system using
dual row/column decoders and read/write circuits.

[0042]FIG. 13 is a block diagram depicting one embodiment of a sense
block.

[0043]FIG. 14 depicts an example of an organization of a memory array into
blocks for odd-even and all bit line memory architectures.

[0044]FIG. 15 depicts an example set of threshold voltage distributions
with single pass programming.

[0045]FIG. 16 depicts an example set of threshold voltage distributions
with multi-pass programming.

[0046]FIGS. 17a-c show various threshold voltage distributions and
describe a process for programming non-volatile memory.

[0047]FIG. 18 is a flow chart describing one embodiment of a process for
programming non-volatile memory.

[0048]FIG. 19 depicts an example pulse train applied to the control gates
of non-volatile storage elements during programming.

DETAILED DESCRIPTION

[0049]The present invention provides a method for sensing a programming
condition of non-volatile storage elements using all bit line sensing.

[0050]One example of a memory system suitable for implementing the present
invention uses the NAND flash memory structure, which includes arranging
multiple transistors in series between two select gates. The transistors
in series and the select gates are referred to as a NAND string. FIG. 1
is a top view showing one NAND string. FIG. 2 is an equivalent circuit
thereof. The NAND string depicted in FIGS. 1 and 2 includes four
transistors, 100, 102, 104 and 106, in series and sandwiched between a
first select gate 120 and a second select gate 122. Select gate 120 gates
the NAND string connection to bit line 126. Select gate 122 gates the
NAND string connection to source line 128. Select gate 120 is controlled
by applying the appropriate voltages to control gate 120CG. Select gate
122 is controlled by applying the appropriate voltages to control gate
122CG. Each of the transistors 100, 102, 104 and 106 has a control gate
and a floating gate. Transistor 100 has control gate 100CG and floating
gate 100FG. Transistor 102 includes control gate 102CG and floating gate
102FG. Transistor 104 includes control gate 104CG and floating gate
104FG. Transistor 106 includes a control gate 106CG and floating gate
106FG. Control gate 100CG is connected to word line WL3, control gate
102CG is connected to word line WL2, control gate 104CG is connected to
word line WL1, and control gate 106CG is connected to word line WL0. The
control gates can also be provided as portions of the word lines. In one
embodiment, transistors 100, 102, 104 and 106 are each storage elements,
also referred to as memory cells. In other embodiments, the storage
elements may include multiple transistors or may be different than that
depicted in FIGS. 1 and 2. Select gate 120 is connected to select line
SGD (drain select gate). Select gate 122 is connected to select line SGS
(source select gate).

[0051]FIG. 3 is a circuit diagram depicting three NAND strings. A typical
architecture for a flash memory system using a NAND structure will
include several NAND strings. For example, three NAND strings 320, 340
and 360 are shown in a memory array having many more NAND strings. Each
of the NAND strings includes two select gates and four storage elements.
While four storage elements are illustrated for simplicity, modern NAND
strings can have up to thirty-two or sixty-four storage elements, for
instance.

[0052]For example, NAND string 320 includes select gates 322 and 327, and
storage elements 323-326, NAND string 340 includes select gates 342 and
347, and storage elements 343-346, NAND string 360 includes select gates
362 and 367, and storage elements 363-366. Each NAND string is connected
to the source line by its select gates (e.g., select gates 327, 347 or
367). A selection line SGS is used to control the source side select
gates. The various NAND strings 320, 340 and 360 are connected to
respective bit lines 321, 341 and 361, by select transistors in the
select gates 322, 342, 362, etc. These select transistors are controlled
by a drain select line SGD. In other embodiments, the select lines do not
necessarily need to be in common among the NAND strings; that is,
different select lines can be provided for different NAND strings. Word
line WL3 is connected to the control gates for storage elements 323, 343
and 363. Word line WL2 is connected to the control gates for storage
elements 324, 344 and 364. Word line WL1 is connected to the control
gates for storage elements 325, 345 and 365. Word line WL0 is connected
to the control gates for storage elements 326, 346 and 366. As can be
seen, each bit line and the respective NAND string comprise the columns
of the array or set of storage elements. The word lines (WL3, WL2, WL1
and WL0) comprise the rows of the array or set. Each word line connects
the control gates of each storage element in the row. Or, the control
gates may be provided by the word lines themselves. For example, word
line WL2 provides the control gates for storage elements 324, 344 and
364. In practice, there can be thousands of storage elements on a word
line.

[0053]Each storage element can store data. For example, when storing one
bit of digital data, the range of possible threshold voltages (VTH)
of the storage element is divided into two ranges which are assigned
logical data "1" and "0." In one example of a NAND type flash memory, the
VTH is negative after the storage element is erased, and defined as
logic "1." The VTH after a program operation is positive and defined
as logic "0." When the VTH is negative and a read is attempted, the
storage element will turn on to indicate logic "1" is being stored. When
the VTH is positive and a read operation is attempted, the storage
element will not turn on, which indicates that logic "0" is stored. A
storage element can also store multiple levels of information, for
example, multiple bits of digital data. In this case, the range of
VTH value is divided into the number of levels of data. For example,
if four levels of information are stored, there will be four VTH
ranges assigned to the data values "11", "10", "01", and "00." In one
example of a NAND type memory, the VTH after an erase operation is
negative and defined as "11". Positive VTH values are used for the
states of "10", "01", and "00." The specific relationship between the
data programmed into the storage element and the threshold voltage ranges
of the element depends upon the data encoding scheme adopted for the
storage elements. For example, U.S. Pat. No. 6,222,762 and U.S. Patent
Application Pub. 2004/0255090, both of which are incorporated herein by
reference in their entirety, describe various data encoding schemes for
multi-state flash storage elements.

[0054]Relevant examples of NAND type flash memories and their operation
are provided in U.S. Pat. Nos. 5,386,422, 5,522,580, 5,570,315,
5,774,397, 6,046,935, 6,456,528 and 6,522,580, each of which is
incorporated herein by reference.

[0055]When programming a flash storage element, a program voltage is
applied to the control gate of the storage element and the bit line
associated with the storage element is grounded. Electrons from the
channel are injected into the floating gate. When electrons accumulate in
the floating gate, the floating gate becomes negatively charged and the
VTH of the storage element is raised. To apply the program voltage
to the control gate of the storage element being programmed, that program
voltage is applied on the appropriate word line. As discussed above, one
storage element in each of the NAND strings share the same word line. For
example, when programming storage element 324 of FIG. 3, the program
voltage will also be applied to the control gates of storage elements 344
and 364.

[0056]FIG. 4 depicts a cross-sectional view of an NAND string formed on a
substrate. The view is simplified and not to scale. The NAND string 400
includes a source-side select gate 406, a drain-side select gate 424, and
eight storage elements 408, 410, 412, 414, 416, 418, 420 and 422, formed
on a substrate 490. A number of source/drain regions, one example of
which is source drain/region 430, are provided on either side of each
storage element and the select gates 406 and 424. In one approach, the
substrate 490 employs a triple-well technology which includes a p-well
region 492 within an n-well region 494, which in turn is within a p-type
substrate region 496. The NAND string and its non-volatile storage
elements can be formed, at least in part, on the p-well region. A source
supply line 404 with a potential of VSOURCE is provided in addition
to a bit line 426 with a potential of VBL. In one possible approach,
a voltage can be applied to the p-well region 492 via a terminal 402. A
voltage can also be applied to the n-well region 494 via a terminal 403.

[0057]During a read or verify operation, including an erase-verify
operation, in which the condition of a storage element, such as its
threshold voltage, is ascertained, VCGR is provided on a selected
word line which is associated with a selected storage element. Further,
recall that the control gate of a storage element may be provided as a
portion of the word line. For example, WL0, WL1, WL2, WL3, WL4, WL5, WL6
and WL7 can extend via the control gates of storage elements 408, 410,
412, 414, 416, 418, 420 and 422, respectively. A read pass voltage,
VREAD, can be applied to unselected word lines associated with NAND
string 400, in one possible boosting scheme. Other boosting schemes apply
VREAD to some word lines and lower voltages to other word lines.
VSGS and VSGD are applied to the select gates 406 and 424,
respectively.

[0058]FIGS. 5a-d depict programming of a non-volatile storage element. In
one possible programming technique, a lower page, middle page and upper
page are programmed in three steps as depicted at FIGS. 5a, 5b and 5c,
respectively. When programming the lower page of data after an erase
operation, two VTH distributions 510 and 512 are provided. The
lowest distribution 510 represents the erased state and has a negative
VTH. Next, the first and second VTH distributions 520 and 522,
respectively, of FIG. 5b are obtained from the first VTH
distribution 510 of FIG. 5a, and the third and fourth VTH
distributions 524 and 526, respectively, of FIG. 5b are obtained from the
second VTH distribution 512 of FIG. 5a. The first and second
VTH distributions of FIG. 5c, representing the final erased state E
and a first programmed state A, respectively, are obtained from the first
VTH distribution 520 of FIG. 5b. The third and fourth VTH
distributions of FIG. 5c, representing the second and third programmed
states B and C, respectively, are obtained from the second VTH
distribution 522 of FIG. 5b. The fifth and sixth VTH distributions
of FIG. 5c, representing the fourth and fifth programmed states D and E,
respectively, are obtained from the third VTH distribution 524 of
FIG. 5b. The seventh and eight VTH distributions of FIG. 5c,
representing the sixth and seventh programmed states F and G,
respectively, are obtained from the fourth VTH distribution 526 of
FIG. 5b. Further, code words 111, 011, 001, 101, 100, 000, 010 and 110
may be associated with the states E, A, B, C, D, E, F and G,
respectively.

[0059]States E and A are examples of negative threshold voltage states.
Depending on the implementation, one or more states can be negative
threshold voltage states.

[0060]FIG. 5c also depicts verify voltages which are used to obtain the
distributions indicated. Specifically, verify voltages VVE,
VVA, VVB, VVC, VVD, VVE, VVF and VVG
are associated with distributions E, A, B, C, D, E, F and G,
respectively. During programming, the threshold voltages of storage
elements which are to be programmed to a given distribution are compared
to the associated verify voltage. The storage elements receive
programming pulses via an associated word line until their threshold
voltage is verified to have exceeded the associated verify voltage.

[0061]FIG. 5d depicts read voltages which are used to read the programming
state of a storage element. Once the storage elements have been
programmed, they can subsequently be read using read voltages VRA,
VRB, VRC, VRD, VRE, VRF and VRG. One or
more storage elements, typically associated with a common word line, are
compared to each read voltage to determine whether their threshold
voltage exceeds the read voltage. The state of the storage element can
then be determined by the highest read voltage which is exceeded. The
read voltages are provided between the neighboring states.

[0062]Note that the programming process depicted is one possible example
as other approaches are possible.

Current Sensing Of Negative Threshold Voltage

[0063]In non-volatile storage devices, including those using NAND memory
designs, a satisfactory methodology has not been available to use current
sensing for sensing negative threshold voltage states of non-volatile
storage elements during read or verify operations. Voltage sensing has
been used but has been found to take a long time to complete. Further,
due to bit line-to-bit line capacitive coupling and other effects,
voltage sensing has been unsuitable for all bit line sensing, in which
sensing is performed on a group of adjacent storage elements
concurrently. One possible solution involves regulating the source
voltage and p-well voltage to some fixed, positive DC level during
sensing when using current sensing, and connecting the control gate of
the sensed storage element, via its associated word line, to a lower
potential than the source and p-well voltage. It is also possible for the
source voltage and p-well voltage to differ. With this methodology of
combining biasing of the source and the p-well to some fixed potential,
it is possible to sense one or more negative threshold voltage states
using current sensing. Further, current sensing is compatible with all
bit line sensing since it avoids many of the disadvantages of voltage
sensing.

[0064]FIG. 6a depicts a configuration of a NAND string and components for
sensing. In a simplified example, a NAND string 612 includes four storage
elements which are in communication with word lines WL0, WL1, WL2 and
WL3, respectively. In practice, additional storage elements and word
lines can be used. Further, additional NAND strings are typically
arranged adjacent to one another in a block or other set of non-volatile
storage elements (see, e.g., FIG. 14). The storage elements are coupled
to a p-well region of a substrate. A bit line 610 having a voltage
VBL is depicted, in addition to sense components 600. In particular,
a BLS (bit line sense) transistor 606 is coupled to the bit line 610. The
BLS transistor 606 is a high voltage transistor, and is opened in
response to a control 608 during sense operations. A BLC (bit line
control) transistor 604 is a low voltage transistor which is opened in
response to the control 608 to allow the bit line to communicate with a
current sensing module 602. During a sense operation, such as a read or
verify operation, a pre-charge operation occurs in which a capacitor in
the current sensing module 602 is charged. The BLC transistor 604 may be
opened to allow the pre-charging. Also during the sense operation, for a
storage element having a negative threshold voltage state, a positive
voltage is applied to a word line of one or more storage elements
involved in the operation. The use of a positive voltage for the selected
word line in a sensing operation in which a negative threshold voltage is
sensed is advantageous since a negative charge pump is not needed to
provide a negative word line voltage. The incorporation of a negative
charge pump into many non-volatile storage systems would require
substantial process study and modification.

[0065]For example, assume the selected word line is WL1. The voltage on
WL1 is coupled to the control gates of the storage elements on the word
line as the control gate read voltage VCGR. Further, a positive
voltage VSOURCE can be applied to the source side of the NAND string
630 and a positive voltage VP-WELL can be applied to the p-well.
VSOURCE and VP-WELL are greater than VCGR, in one
implementation. VSOURCE and VP-WELL can differ from one
another, or they can be coupled to the same DC voltage, VDC.
Further, VDC>VCGR. As an example, VDC can be in the
range of about 0.4 to 1.5 V, e.g., 0.8 V. A higher VDC allows
sensing of more negative threshold voltage states. For example, first and
second negative threshold voltage states VTH1=-1.0 V and
VTH2=-0.5 V might be sensed using VDC=1.5 V and VDC=1.0 V,
respectively. VDC can be set at a level such that
VDC-VTH>0V. Generally, for sensing a negative threshold
voltage, the word line and source voltages are set so that the
gate-to-source voltage is less than zero, i.e., VGS<0 V. The
selected storage element is conductive if the gate-to-source voltage is
greater than the storage element's threshold voltage, i.e.,
VGS>VTH. For sensing a positive threshold voltage, the
source and p-well can be kept at the same voltage while the selected word
line voltage is adjusted.

[0066]At the drain side of the NAND string 630, the BLS transistor 610 is
turned on, e.g., made conductive or opened. Additionally, a voltage
VBLC is applied to the BLC transistor 600 to make it conductive. The
pre-charged capacitor in the current sensing module 602 discharges
through the bit line and into the source so that the source acts as a
current sink. The pre-charged capacitor at the drain of the NAND string
may be pre-charged to a potential which exceeds a potential of the source
so that a current flows through the selected non-volatile storage element
and sinks into the source when the selected storage element is in the
conductive state.

[0067]In particular, if the selected storage element is in a conductive
state due to the application of VCGR, a relatively high current will
flow. If the selected storage element is in a non-conductive state, no or
relatively little current will flow. The current sensing module 602 can
sense the cell/storage element current, ICELL. In one possible
approach, the current sensing module determines a voltage drop which is
tied to a fixed current flow by the relationship ΔV=it/C, where
ΔV is the voltage drop, i is the fixed current, t is a
predetermined discharge time period and C is the capacitance of the
pre-charged capacitor in the current sensing module. See also FIG. 6d,
which depicts voltage drop with time for different lines of fixed
current. The greater voltage drops represent higher currents. At the end
of a given discharge period, since i and C are fixed, ΔV for a
given current can be determined. In one approach, a p-mos transistor is
used to determine a level of ΔV relative to a demarcation value. In
another possible approach, a cell current discriminator serves as a
discriminator or comparator of current levels by determining whether the
conduction current is higher or lower than a given demarcation current.

[0068]Voltage sensing, in contrast, does not involve sensing a voltage
drop which is tied to a fixed current. Instead, voltage sensing involves
determining whether charge sharing occurs between a capacitor in a
voltage sensing module and a capacitance of the bit line. Current is not
fixed or constant during the sensing. Little or no charge sharing occurs
when the selected storage element is conductive, in which case the
voltage of the capacitor in the voltage sensing module does not drop
significantly. Charge sharing does occur when the selected storage
element is non-conductive, in which case the voltage of the capacitor in
the voltage sensing module does drop significantly.

[0069]The current sensing module 602 thus can determine whether the
selected storage element is in a conductive or non-conductive state by
the level of current. Generally, a higher current will flow when the
selected storage element is in a conductive state and a lower current
will flow when the selected storage element is in a non-conductive state.
A threshold voltage of the selected storage element is above or below a
compare level, such as a verify level (see FIG. 5c) or a read level (see
FIG. 5d), when it is in a non-conductive state or a conductive state,
respectively.

[0070]FIG. 6b depicts waveforms associated with FIG. 6a. Waveforms 620
depict VSOURCE and VP-WELL, VBL and VBLC.
VSOURCE and VP-WELL are set at an elevated level at t1 during
the sense operation. In one approach, such as when the sense operation
involves a negative threshold voltage, VSOURCE and VP-WELL
exceed VCGR. However, VSOURCE and VP-WELL need not exceed
VCGR, e.g., when the sense operation involves a positive threshold
voltage. VBL increases with VSOURCE between t1 and t2. At t2,
the pre-charged capacitor is discharged, increasing VBL. Thus, a
potential of a drain (e.g., VBL) associated with the selected
non-volatile storage element is above a potential of a source (e.g.,
VSOURCE) associated with the selected non-volatile storage element.
VBLC tracks VBL but is slightly higher due to the threshold
voltage of the BLC transistor. In practice, after rising, VBL will
drop slightly (not shown) if current flows in the NAND string. For
example, VBL may rise to 1 V when VBLC=2 V and the threshold
voltage of the BLC transistor is 1 V. When sensing, if current flows,
VBL may drop from 1 V to 0.9 V, for instance. Waveform 622 depicts a
voltage applied to the BLS transistor, indicating it is conductive
between t0 and t5. Waveform 624 depicts a sense signal which is a control
signal indicating the time t after the capacitor starts discharging in
the current sensing module.

[0071]Waveforms 626 and 628 depict a sensed voltage which is tied to a
fixed current for the selected bit line. A determination can be made at
t3 as to whether the voltage exceeds a demarcation level. It can be
concluded that the selected storage element is conductive when the
voltage drops below the demarcation level (e.g., line 628). The selected
storage element is non-conductive if the voltage does not drop below the
demarcation level (e.g., line 626).

[0072]FIG. 6c depicts a sensing process associated with FIGS. 6a and 6b.
An overview of the sensing process is provided. In this and the other
flowcharts, the steps depicted do not necessary occur as discrete steps
and/or in the sequence depicted. A sense operation, such as a read or
verify operation, begins at step 640. Step 642 includes opening the BLS
and BLC transistors to pre-charge the bit line. Step 644 includes setting
the word line voltages. Step 646 includes setting VSOURCE and
VP-WELL. Step 648 includes determining whether the storage element
is conductive or non-conductive using current sensing. If another sense
operation is to be performed, at decision step 650, the control flow
continues at step 640. Otherwise, the process ends at step 652.

[0073]Multiple sensing operations can be performed successively, for
example, one for each verify or read level. In one approach, the same
source and p-well voltages are applied in each sense operation, but the
selected word line voltage is changed. Thus, in a first sensing
operation, a first voltage can be applied to the control gate/word line
of a selected storage element, the source voltage applied to the source,
and the p-well voltage applied to the p-well. A determination is then
made as to whether the storage element is in a conductive state or a
non-conductive state using current sensing while applying the first
voltage and the source voltage. A second sensing operation includes
applying a second voltage to the control gate while applying the same
source and p-well voltages. A determination is then made as to whether
the storage element is in a conductive state or a non-conductive state.
Successive sensing operations similarly can vary the selected word line
voltage while using the same source and p-well voltages.

[0074]Further, sensing can be performed concurrently for multiple storage
elements which are associated with a common word line and source. The
multiple storage elements may be in adjacent or non-adjacent NAND
strings. All bit line sensing, discussed previously, involves concurrent
sensing of storage elements in adjacent NAND strings. In this case, the
sensing includes determining, in concurrent sensing operations, whether
each of the non-volatile storage elements is in the conductive or
non-conductive state using current sensing.

Current Sensing With Biasing Of Source And P-Well

[0075]In non-volatile storage devices including those using NAND memory
designs, current sensing can be used for sensing threshold voltage states
of non-volatile storage elements during read or verify operations.
However, such current sensing has resulted in variations or "bouncing" of
a source voltage, especially at a ground voltage. The degree of the
bouncing depends on the level of current through the storage elements.
Moreover, the bouncing can result in sensing errors. One method of
controlling cell source bounce during sensing is to sense using at least
two strobes. This can minimize the effect of cell source bounce. For
example, with current sensing, a current in the NAND string of the
selected storage element may be sensed at each strobe from a control. A
relatively high or otherwise inaccurate bounce current may be present at
the first strobe, while a lower current is present by the time of the
second strobe, where the lower current more accurately represents the
sensed state of the storage element. However, the need to use an
additional strobe to wait for the current to settle down consumes
additional current and sense time. See FIG. 7a, which depicts variations
in current and voltage with time due to ground bounce during a sense
operation.

[0076]Another technique is to couple the source to the gate and drain of
the storage elements. However, this technique is complicated, requiring
extra circuits, and causing some impact on die size and power consumption
of the memory chip. Moreover, this technique might not work very well due
to RC delays from the source to the gate of the storage elements.

[0077]One approach which generally avoids these disadvantages is to
regulate the source and p-well to some fixed positive DC level, instead
of to ground, during sensing. By keeping the source and p-well at the
fixed DC level, bouncing in the source voltage is avoided, so we can
sense data using just one strobe. As a result, sense time and power
consumption are reduced. Further, there is no need for significant
additional circuitry, so die size is not adversely impacted. It is also
possible to ground the p-well while regulating the source voltage at a
fixed, positive DC level. Regulating the source voltage to a fixed,
positive DC level can be achieved more readily than regulating the source
voltage to ground since the regulating circuit need only sense positive
voltages. A voltage regulator typically works by adjusting its output
based on a comparison of a monitored level, e.g., of the source, to an
internal reference voltage. If the monitored level drops below the
internal reference voltage, the voltage regulator can increase its
output. Similarly, if the monitored level increases above the internal
reference voltage, the voltage regulator can lower its output. A voltage
regulator may use an op-amp, for instance. However, if the reference
voltage is at ground, the voltage regulator generally cannot lower its
output below 0 V if the monitored level becomes greater than 0 V.
Moreover, the voltage regulator may not be able to distinguish monitored
levels below 0 V. Regulating the source voltage to a fixed, positive DC
level thus avoids ground bounce and can reduce current consumption and
sense time. See FIG. 7b, which depicts reduced variations in current and
voltage with source voltage regulated to a fixed, positive DC level
during a sense operation.

[0078]FIG. 7c depicts another configuration of a NAND string and
components for sensing. The configuration corresponds to that provided in
FIG. 6a except a voltage regulator 720 is depicted. As mentioned, the
source voltage and p-well voltage can be regulated to a fixed, positive
DC level during sense operations.

[0079]During a sense operation, such as a read or verify operation, of a
storage element, a voltage is applied to a word line of one or more
storage elements involved in the operation. For example, assume the
selected word line is WL1. This voltage is coupled to the control gates
of the storage elements on the word line as the control gate read voltage
VCGR. Further, a fixed DC voltage can be applied to the source side
of the NAND string 612 as well as to the p-well as the source voltage
VSOURCE and the p-well voltage VP-WELL, respectively. When the
threshold voltage is negative, VCGR can be positive, and
VSOURCE and VP-WELL can be greater than VCGR, in one
implementation. When the threshold voltage is positive, VCGR can be
greater than VSOURCE and VP-WELL, in one implementation.
VSOURCE and VP-WELL can differ from one another, or they can be
coupled to the same DC voltage, VDC. As an example, VDC can be
regulated by the voltage regulator 720 to be in the range of about 0.4 to
1.2 V, e.g., 0.8 V. Due to the constant voltage on the source and p-well,
accurate sensing can be achieved by using just one strobe, as discussed
previously. Further, all bit line sensing can be performed in which
storage elements associated with all bit lines are sensed (see FIG. 14).
In particular, the voltage regulator 720 can receive a reference voltage,
VREF,SOURCE which is used for regulating VSOURCE to a level
greater than 0 V, and a reference voltage VREF,P-WELL, which is used
for regulating the p-well voltage to a level greater than or equal to 0
V.

[0080]FIG. 7d depicts a sensing process associated with FIGS. 7a-7c. A
sense operation, such as a read or verify operation, begins at step 700.
Step 702 includes opening the BLS and BLC transistors and pre-charging
the bit line. Step 704 includes setting the word line voltages. Step 706
includes regulating VSOURCE and VP-WELL to positive DC levels.
Step 708 includes sensing whether the selected storage element is
conductive or non-conductive using current sensing. If there is another
sense operation, at decision step 710, the control flow continues at step
700. Otherwise, the process ends at step 712.

[0081]Further, as discussed previously, sensing can be performed
concurrently for multiple storage elements which are associated with a
common word line and source. The multiple storage elements may be in
adjacent or non-adjacent NAND strings. In this case, the sensing includes
determining, in concurrent sensing operations, whether each of the
non-volatile storage elements is in the conductive or non-conductive
state using current sensing. For each sensing operation, the voltages are
regulated as discussed.

Source Bias All Bit Line Sensing

[0082]All bit line sensing involves performing sensing operations on
storage elements in adjacent NAND strings (see FIG. 14). One potential
sensing approach uses a DC storage element current to discharge charges
on a fixed capacitance in a sensing module in a fixed period of time to
convert the storage element's threshold voltage value into a digital data
format. However, this requires a relatively large current sink into the
source side of the NAND string. Further, as discussed previously, for
sensing negative threshold voltage values, a bias can be applied to both
the source and the p-well using an analog voltage level to avoid the need
for negative word line voltages and a negative charge pump. However,
since all bit line sensing is very sensitive to the source bias level, to
maintain the analog voltage level requires a relatively large voltage
regulator and an even distribution of the source voltage into the array.
This can increase the require device area.

[0083]Another approach to all bit line sensing uses voltage sensing,
discussed previously. This approach does not require a large voltage
regulator because there is no DC current to the source side. However,
this approach has not been able to successfully sense every bit line at
the same time due to bit line-to-bit line coupling noise. Instead, only
every alternative bit line is sensed at a given time, e.g., in odd-even
sensing (see FIG. 14). Therefore, the performance in terms of sensing
time has not been optimal. In particular, all bit line sensing has been
problematic due to the close proximity of the adjacent NAND strings.
Capacitive coupling can occur especially from NAND strings in which the
selected storage element is conductive to NAND string in which the
selected storage element is non-conductive. The bit line voltage of the
NAND string in which the selected storage element is non-conductive is
thereby increased, interfering with the sensing operation. This
capacitive coupling is depicted by a capacitance to neighboring bit lines
813. The neighboring bit lines/NAND strings can be immediately adjacent
or non-adjacent. Capacitive coupling from adjacent bit lines/NAND strings
is strongest but some capacitively coupling from non-adjacent bit
lines/NAND strings can also occur. A capacitance to ground 811 is also
depicted.

[0084]To overcome these issues, sensing can be performed using a scheme as
depicted in FIG. 8a. FIG. 8a depicts a configuration of a NAND string and
components, including a current discharge path. In a simplified example,
a NAND string 812 includes four storage elements which are in
communication with word lines WL0, WL1, WL2 and WL3, respectively. In
practice, additional storage elements and word lines can be used.
Further, additional NAND strings are typically arranged adjacent to one
another in a block or other set of non-volatile storage elements. The
storage elements are coupled to a p-well region of a substrate. A bit
line 810 having a voltage VBL is depicted, in addition to sense
components 800. In particular, a BLS (bit line sense) transistor 806,
which is initially open or conductive, is coupled to the bit line 810 via
a sense node 814. The BLS transistor 806 is a high voltage transistor,
and is made conductive in response to a control 808 during sense
operations. A BLC (bit line control) transistor 804, which is
non-conductive initially, is a low voltage transistor which is opened in
response to the control 808 to allow the bit line to communicate with a
voltage sensing module/circuit 802. During a sense operation, such as a
read or verify operation, a pre-charge operation occurs in which a
capacitor in the voltage sensing module 602 is charged. The BLC
transistor 804 may be opened to allow the pre-charging.

[0085]Further, a relatively weak current pull down device is introduced.
In particular, a path 816, which is part of a current discharge path for
the NAND string 812, is coupled to the sense node 814, which in turn is
coupled to the bit line 810. A transistor, referred to as GRS transistor
818, is provided in a conductive state so that the path 816 is coupled to
a path 820, which is also part of the current discharge path. A current
source 825, e.g., current mirror, which provides a current of IREF
is provided in parallel to the paths 816, 820 to pull the current
iCELL on the paths down to ground. In an example, a relatively weak
pull down is provided with an IREF of about 150 nA. However, the
strength of the current source 825 can vary according to the specific
implementation.

[0086]In one possible configuration, the current source 825 is common to
multiple bit lines and NAND strings. In this case, a transistor 824
couples the current source 825 to the different NAND strings. A path 822
carries a control signal for the GRS transistor 818, which is local to a
particular bit line and NAND string, while a path 826 is a common ground
path for multiple bit lines.

[0087]During sensing, the bit line will charge up to a level which is
based on the threshold voltage of the selected storage element, and a
body effect. With a negative Vti, the storage element will be conductive
even with a VGCR=0 V. VP-WELL may be set to 0 V.

[0088]The transistors 818 and 824 are made conductive to create a current
discharge path and pull down which serves to discharge any charge that is
coupled to the NAND string 812 from one or more neighboring NAND strings
due the capacitance to the neighboring bit line 813. Any extra charge
that is generated by a neighboring bit line's coupling noise will
therefore die out eventually. After a certain amount of time, all the bit
lines reach their DC levels, and the BLC transistor 804 is turned on to
allow charge sharing between the voltage sensing module 802 and the sense
node 814 so that voltage sensing of the threshold voltage of the selected
storage element can occur. The voltage sensing module 802 may perform
voltage sensing as part of a reading or verifying operation, for
instance.

[0089]FIG. 8b depicts a configuration of the NAND string and components of
FIG. 8a, when voltage sensing occurs. Here, the BLC transistor 804 is
opened so that current flows from the voltage sensing module 802 toward
the discharge path in addition to the current being discharged from the
NAND string 812. Thus, the GRS transistor remains in the conductive state
so that discharging continues during the voltage sensing.

[0090]FIG. 8c depicts waveforms associated with FIGS. 8a and 8b.
VSOURCE is depicted at waveform 830, and voltages on three adjacent
bit lines BL0, BL1 and BL2 are depicted at waveforms 832, 834 and 836,
respectively. A voltage VBLS on the BLS transistor is depicted at
waveform 838, a voltage VBLC on the BLC transistor is depicted at
waveform 840, and a voltage VGRS on the GRS transistor is depicted
at waveform 842. A sensed voltage on BL0 and BL2 is depicted at waveform
844. A sensed voltage on BL1 is depicted at waveform 846 when the
selected storage element on BL1 is conductive and at waveform 848 when
the selected storage element on BL1 is non-conductive. As mentioned,
during voltage sensing, charge sharing between the voltage sensing module
and the bit line occurs when the selected storage element is
non-conductive. This charge sharing lowers the sensed voltage at the
voltage sensing module. Little or no charge sharing between the voltage
sensing module and the bit line occurs when the selected storage element
is conductive so that the sensed voltage at the voltage sensing module
remains high. The sensed voltages at other times are not depicted as
sensing does not occur.

[0091]At t0, VBLS increases so that the BLS transistor is conductive.
At t1, VSOURCE is applied as a common source voltage for a set of
NAND strings. In this example, we assume that the selected storage
element associated with BL1 is non-conductive while the selected storage
elements associated with BL0 and BL2 are conductive. BL0 is adjacent to
BL1 on one side and BL2 is adjacent to BL1 on the other side (See FIG.
14). With the increase in VSOURCE at t1, VBL0 and VBL2
will rise as depicted by waveforms 832 and 836, respectively, causing
capacitive coupling to BL1, as depicted by the transient increase in
VBL1. This coupling will substantially die out by t2. The GRS
transistor for BL1 remains conductive between t1 and t5 to allow the bit
line to discharge the coupled charge, as discussed.

[0092]At t3, the BLC transistor is opened by increasing VBLC as
depicted by waveform 840, thereby allowing sensing to occur for the
selected storage element on BL1. Note that corresponding components
associated with BL0, BL2 and other bit lines can be controlled similarly
to allow sensing to occur concurrently on those other bit lines. For BL1,
the sensed voltage at the voltage sensing module will drop as depicted by
waveform 846 if the selected storage element is non-conductive. On the
other hand, the sensed voltage will remain generally high as depicted by
waveform 844 if the selected storage element is conductive. The voltage
sensing components may use a voltage break point at a specified sense
time t4 to determine whether the selected storage element is conductive
or non-conductive. As mentioned, if the sensed voltage exceeds the
breakpoint, this indicates the storage element is open, while if the
sensed voltage drops below the breakpoint, this indicates the storage
element is non-conductive. VSOURCE is lowered at t5 and the BLS
transistor is non-conductive at t6, indicating the end of the sense
operation. VP-WELL may be set at 0 V during the sensing, in one possible
approach. The selected word line receives VCGR while the unselected
word lines can receive read pass voltages according to the particular
sensing scheme.

[0093]Thus, after the source voltage is applied at t1, a predetermine
delay of duration t3-t1 is instituted to allow sufficient time for the
capacitive coupling from neighboring bit lines to be fully or at least
partly discharged. The appropriate delay can be set as required for
particular implementations based on theoretical and/or experimental
tests. After the delay, voltage sensing occurs. At the specified time t4,
a determination is made as to whether the storage element is in a
conductive or non-conductive state and, therefore, has a threshold
voltage which is below or above, respectively, a verify or read compare
level.

[0094]FIG. 8d depicts a sensing process associated with FIGS. 8a-8c. At
step 850, a sense operation begins. At step 852, the BLS transistor is
opened while the BLC transistor remains non-conductive, and the bit line
is pre-charged. At step 854, the word line voltages are set. At step 856,
VSOURCE and VP-WELL are set (VP-WELL=0 V). At step 858,
bit line discharges. At step 860, the BLC transistor is made conductive
to allow sensing to occur. At step 862, a determination is made as to
whether the selected storage element is conductive or non-conductive
using voltage sensing. If there is another sense operation, at decision
step 864, the control flow continues at step 850. Otherwise, the process
ends at step 868.

[0095]Further, as discussed previously, sensing can be performed
concurrently for multiple storage elements which are associated with a
common word line and source. The multiple storage elements may be in
adjacent or non-adjacent NAND strings. In this case, the sensing includes
determining, in concurrent sensing operations, whether each of the
non-volatile storage elements is in the conductive or non-conductive
state using current sensing. The delay before the BLC transistor is
opened can be instituted for each NAND string so that the NAND strings
can discharge as needed before sensing occurs.

Temperature Compensating Bit Line During Sense Operations

[0096]In present non-volatile storage devices, such as NAND flash memory
devices, temperature variations present various issues in reading and
writing data. A memory device is subject to varying temperatures based on
the environment in which it is located. For example, some current memory
devices are rated for use between -40° C. and +85° C.
Devices in industrial, military and even consumer applications may
experience significant temperature variations. Temperature affects many
transistor parameters, the dominant among which is the threshold voltage.
In particular, temperature variations can cause read errors and widen the
threshold voltage distributions of the different states of a non-volatile
storage element. An improved technique for addressing temperature effects
in non-volatile storage devices is discussed below.

[0097]FIG. 9a depicts a NAND string and components for
temperature-compensated sensing. Like numbered components correspond to
those provided in FIG. 8a. The current discharge path of FIG. 8a is not
depicted here. However, it is possible for the configuration of FIG. 8a
to be combined with the configuration of FIG. 9a or some of the other
figures provided herein. In addition, a temperature-dependent circuit 900
is provided as part of the control 808 to provide a temperature
compensated voltage to the BLC transistor 804. The BLC transistor 804 has
one node which is coupled to the voltage sensing module 802 and another
node which is coupled to a drain or bit line which is associated with the
NAND string 812 or other set of non-volatile storage elements.

[0098]During a sense operation, a voltage VBLC is applied to the BLC
transistor 600, which couples the bit line or drain side of the NAND
string 812 to the voltage sense module 802. In accordance with the
approach herein, VBLC is set based on temperature to cancel out, or
compensate for, variations in VBL with temperature. Specifically,
VBLC=VBL+VTH (temperature-independent)+ΔV, where
ΔV is a voltage change due to temperature. VBL is also changed
by ΔV due to temperature. Thus, VBLC can be controlled so that
it varies with temperature in accordance with the variations in VBL.
In particular, ΔV on the bit line can be matched to the ΔV of
VBLC by using the temperature-dependent circuit 900. A current
iCELL flows in the NAND string 812. The dotted line denotes charge
sharing.

[0099]FIG. 9b illustrates a threshold voltage change with temperature,
e.g., ΔVTH/° C. Typically, the threshold voltage of a
non-volatile storage element decreases as temperature increases. The
change in voltage relative to the change in temperature can be expressed
in terms of a temperature coefficient which is typically about -2
mV/° C. The temperature coefficient depends on various
characteristics of the memory device, such as doping, layout and so
forth. Moreover, the temperature coefficient is expected to increase in
magnitude as memory dimensions are reduced.

[0100]Various techniques are known for providing temperature-compensated
signals generally. One or more of these techniques can be used in the
temperature-dependent circuit 900, for instance. Most of these techniques
do not rely on obtaining an actual temperature measurement, although this
approach is also possible. For example, U.S. Pat. No. 6,801,454, titled
"Voltage Generation Circuitry Having Temperature Compensation,"
incorporated herein by reference, describes a voltage generation circuit
which outputs read voltages to a non-volatile memory based on a
temperature coefficient. The circuit uses a band gap current which
includes a temperature-independent portion and a temperature-dependent
portion which increases as temperature increases. U.S. Pat. No.
6,560,152, titled "Non-Volatile Memory With Temperature-Compensated Data
Read", incorporated herein by reference, uses a bias generator circuit
which biases a voltage which is applied to a source or drain of a data
storage element. U.S. Pat. No. 5,172,338, titled "Multi-State EEPROM Read
and Write Circuits and Techniques", incorporated herein by reference,
describes a temperature-compensation technique which uses reference
storage cells that are formed in the same manner as data storage cells
and on the same integrated circuit chip. The reference storage cells
provide reference levels against which measured currents or voltages of
the selected cells are compared. Temperature compensation is provided
since the reference levels are affected by temperature in the same manner
as the values read from the data storage cells. Any of the these
techniques, as well as any other known techniques, can be used to provide
a temperature-compensated voltage to a bit line control line as described
herein.

[0101]VBLC, as discussed, is a voltage of a control signal or voltage
provided to the BLC transistor 804, which allows a sense component to
sense the VTH of a selected storage element which is undergoing an
erase-verify or other sensing operation. The sensing occurs via a bit
line of a NAND string in which the selected storage element is located.
In an example implementation, VBLC=VBL+VTH (BLC
transistor). Thus, the control is configured to increase VBLC with
increasing temperature to track the increase in VBL. For a given
VTH of a storage element, VBL will increase with temperature.

[0102]FIG. 9c illustrates a change in VBLC and VBL with
temperature. The figure depicts how VBLC is increased with
temperature to track the increase in VBL. A control curve which
provides specific changes in VBLC versus temperature can be
programmed into the control 808 according to the specific implementation
based on theoretical and experimental results. Generally, as the VTH
of a storage element decreases with higher temperatures, the bit line
voltage increases. This means VBLC should be higher in order for the
voltage sensing module 802 to sense the higher VBL. Note that the
VTH of the storage element dictates VBL. However, changing
VBLC changes the voltage that the voltage sensing module senses so
that the voltage is temperature compensated. Further, note that changes
in the VTH of the BLC transistor 804 can be cancelled out by
providing a transistor in the temperature-dependent circuit 900 which
varies with temperature similar to the BLC transistor 804.

[0103]FIG. 9d depicts waveforms associated with FIGS. 9a-c. Waveform 910
depicts VSOURCE and VP-WELL, which are set at an elevated level
at t1 during the sense operation. Waveforms 912 and 914 depict an
increase in VBL due to the application of VSOURCE and
VP-WELL. The higher level of VBL at a higher temperature is
depicted by waveform 912 versus waveform 914. In practice, after rising,
VBL may drop slightly (not shown) when current flows in the NAND
string. Waveform 916 depicts a voltage applied to transistor BLS,
indicating that it is turned on at t0. Waveforms 918 and 920 depict
voltages applied to transistor BLC at higher and lower temperatures,
respectively. Note that the waveforms provided are for the temperature
compensation scheme in combination with the scheme of FIGS. 8a-d, where
the opening of the BLC transistor is delayed to allow discharging to
occur before sensing. However, the temperature compensation scheme is not
required to be used in this way, and may be used in other implementations
which do not involve a discharge path and/or a delay in sensing.

[0104]Waveform 922 depicts a sensed voltage in the voltage sensing module
for the selected bit line when the selected storage element is open,
while waveform 924 depicts a sensed voltage when the selected storage
element is non-conductive. A determination can be made at t2 as to
whether the sensed voltage exceeds a breakpoint. It can be concluded that
the selected storage element is conductive or non-conductive when the
sensed voltage exceeds the breakpoint or falls below the breakpoint,
respectively.

[0105]FIG. 9e depicts a sensing process associated with FIGS. 9a-d. A
sense operation, such as a read or verify operation, begins at step 930.
Step 932 includes making the BLS and BLC transistors conductive,
pre-charging the bit line, and setting a temperature dependent VBLC.
Step 934 includes setting the word line voltages, which are optionally
temperature dependent. In one approach, only the selected word line
voltage is temperature-dependent while in other approaches some or all of
the word line voltages are temperature dependent. The word line voltages
can be decreased with increasing temperatures in accordance with the
decrease in VTH (See FIG. 9b). Step 936 includes setting
VSOURCE and VP-WELL. Step 938 includes determining whether the
selected storage element is conductive or non-conductive using voltage
sensing. If another sense operation is to be performed, at decision step
940, the control flow continues at step 930. Otherwise, the process ends
at step 942.

[0106]Note that the drain or bit line of a NAND string communicates with
the drain of the selected storage element since the storage elements on
the drain side of the selected storage element are in a conductive state
due to the sufficiently high voltages on the associated word lines.
Similarly, the source of a NAND string communicates with the source of
the selected storage element since the storage elements on the source
side of the selected storage element are in a conductive state due to the
sufficiently high voltages on the associated word lines. Thus, a voltage
of the drain or bit line of a NAND string is also essentially the voltage
of the drain of the selected storage element, and a voltage of the source
of a NAND string is also essentially the voltage of the source of the
selected storage element. Also, it is not necessary for the storage
element being sensed to be in a NAND string or other set of storage
elements as the technique described herein can be used with a single
storage element.

[0107]Further, as discussed previously, sensing can be performed
concurrently for multiple storage elements which are associated with a
common word line and source.

[0108]Moreover, from the perspective of the control 808, the sensing
process involves receiving information from the temperature-dependent
circuit 900, and, responsive to the information, providing a
temperature-compensated voltage to a control gate of the BLC transistor,
which couples a NAND string or other set of non-volatile storage elements
to a sense circuit. The control can also set the word line, source and
p-well voltages, as well as receive information from the voltage sensing
module 802 regarding the sensed programming condition of the selected
storage element.

[0109]FIG. 9f depicts an erase-verify process. Step 950 includes erasing a
set of storage elements. Step 952 includes beginning soft programming of
one or more of the storage elements to a desired erase state, for
instance. Soft programming generally involved applying voltage pulses to
the selected word line to raise the threshold voltage of one or more the
storage elements on the selected word line. The voltage pulses may be
soft programming pulses which are lower in amplitude than those used for
programming to higher states (step 954). This type of programming may be
used, e.g., when the storage elements undergo a deep erase to ensure that
their threshold voltages are all below the threshold voltage of the
desired erased state. Step 956 includes verifying a programming condition
of the storage elements, e.g., relative to the desired erased state. For
example, this can include performing steps 932-938 of FIG. 9e, discussed
above. If the soft programming is to be continued, at decision step 958,
e.g., when the storage element has not reached the desired erase state,
the control flow continues at step 954. Otherwise, the process ends at
step 960.

[0110]Further, the erase-verify operation can be performed concurrently
for multiple storage elements which are associated with a common word
line and source.

[0111]FIG. 10a illustrates a change in VSOURCE with temperature. In
another approach, VSOURCE is temperature compensated, e.g., so that
it increases with temperature. Generally, VWL=VSOURCE+VTH
(selected storage element), where VWL is the voltage applied to the
selected word line. As discussed, VTH decreases with temperature.
Thus, with VWL fixed, VSOURCE can be set to increase with
temperature to avoid temperature biases during sensing. Further, in one
possible implementation, a constraint may be placed so that VSOURCE
is increased only to positive values. For example, if VSOURCE=0 V at
a baseline temperature, and the temperature increases, VSOURCE
remains at 0 V. If the temperature decreases, VSOURCE increases
according to the temperature coefficient. On the other hand, if
VSOURCE>0 V at a baseline temperature, and the temperature
increases, VSOURCE can decrease to a value which is greater than or
equal to 0 V, i.e., a non-negative value. If the temperature decreases,
VSOURCE increases according to the temperature coefficient.

[0112]FIG. 10b depicts an example of an array of storage elements,
including different sets of NAND strings. Along each column of a memory
array 1000, a bit line 1006 is coupled to the drain terminal 1026 of the
drain select gate for the NAND string 1050. Along each row of NAND
strings, a source line 1004 may connect all the source terminals 1028 of
the source select gates of the NAND strings. An example of a NAND
architecture array and its operation as part of a memory system is found
in U.S. Pat. Nos. 5,570,315; 5,774,397; and 6,046,935.

[0113]The array of storage elements is divided into a large number of
blocks of storage elements. As is common for flash EEPROM systems, the
block is the unit of erase. That is, each block contains the minimum
number of storage elements that are erased together. Each block is
typically divided into a number of pages. A page is a unit of
programming. In one embodiment, the individual pages may be divided into
segments and the segments may contain the fewest number of storage
elements that are written at one time as a basic programming operation.
One or more pages of data are typically stored in one row of storage
elements. A page can store one or more sectors. A sector includes user
data and overhead data. Overhead data typically includes an Error
Correction Code (ECC) that has been calculated from the user data of the
sector. A portion of the controller (described below) calculates the ECC
when data is being programmed into the array, and also checks it when
data is being read from the array. Alternatively, the ECCs and/or other
overhead data are stored in different pages, or even different blocks,
than the user data to which they pertain.

[0114]A sector of user data is typically 512 bytes, corresponding to the
size of a sector in magnetic disk drives. Overhead data is typically an
additional 16-20 bytes. A large number of pages form a block, anywhere
from 8 pages, for example, up to 32, 64, 128 or more pages. In some
embodiments, a row of NAND strings comprises a block.

[0115]Memory storage elements are erased in one embodiment by raising the
p-well to an erase voltage (e.g., 20 V) for a sufficient period of time
and grounding the word lines of a selected block while the source and bit
lines are floating. Due to capacitive coupling, the unselected word
lines, bit lines, select lines, and c-source are also raised to a
significant fraction of the erase voltage. A strong electric field is
thus applied to the tunnel oxide layers of selected storage elements and
the data of the selected storage elements are erased as electrons of the
floating gates are emitted to the substrate side, typically by
Fowler-Nordheim tunneling mechanism. As electrons are transferred from
the floating gate to the p-well region, the threshold voltage of a
selected storage element is lowered. Erasing can be performed on the
entire memory array, separate blocks, or another unit of storage
elements.

[0116]FIG. 11 is a block diagram of a non-volatile memory system using
single row/column decoders and read/write circuits. The diagram
illustrates a memory device 1196 having read/write circuits for reading
and programming a page of storage elements in parallel, according to one
embodiment of the present invention. Memory device 1196 may include one
or more memory die 1198. Memory die 1198 includes a two-dimensional array
of storage elements 1000, control circuitry 1110, and read/write circuits
1165. In some embodiments, the array of storage elements can be three
dimensional. The memory array 1000 is addressable by word lines via a row
decoder 1130 and by bit lines via a column decoder 1160. The read/write
circuits 1165 include multiple sense blocks 1100 and allow a page of
storage elements to be read or programmed in parallel. Typically a
controller 1150 is included in the same memory device 1196 (e.g., a
removable storage card) as the one or more memory die 1198. Commands and
Data are transferred between the host and controller 1150 via lines 1120
and between the controller and the one or more memory die 1198 via lines
1118.

[0117]The control circuitry 1110 cooperates with the read/write circuits
1165 to perform memory operations on the memory array 1000. The control
circuitry 1110 includes a state machine 1112, an on-chip address decoder
1114 and a power control module 1116. The state machine 1112 provides
chip-level control of memory operations. The on-chip address decoder 1114
provides an address interface between that used by the host or a memory
controller to the hardware address used by the decoders 1130 and 1160.
The power control module 1116 controls the power and voltages supplied to
the word lines and bit lines during memory operations.

[0118]In some implementations, some of the components of FIG. 11 can be
combined. In various designs, one or more of the components (alone or in
combination), other than storage element array 1000, can be thought of as
a managing or control circuit. For example, one or more managing or
control circuits may include any one of or a combination of control
circuitry 1110, state machine 1112, decoders 1114/1160, power control
1116, sense blocks 1100, read/write circuits 1165, controller 1150, etc.

[0119]FIG. 12 is a block diagram of a non-volatile memory system using
dual row/column decoders and read/write circuits. Here, another
arrangement of the memory device 1196 shown in FIG. 11 is provided.
Access to the memory array 1000 by the various peripheral circuits is
implemented in a symmetric fashion, on opposite sides of the array, so
that the densities of access lines and circuitry on each side are reduced
by half. Thus, the row decoder is split into row decoders 1130A and 1130B
and the column decoder into column decoders 1160A and 1160B. Similarly,
the read/write circuits are split into read/write circuits 1165A
connecting to bit lines from the bottom and read/write circuits 1165B
connecting to bit lines from the top of the array 1000. In this way, the
density of the read/write modules is essentially reduced by one half. The
device of FIG. 12 can also include a controller, as described above for
the device of FIG. 11.

[0120]FIG. 13 is a block diagram depicting one embodiment of a sense
block. An individual sense block 1100 is partitioned into a core portion,
referred to as a sense module 1180, and a common portion 1190. In one
embodiment, there will be a separate sense module 1180 for each bit line
and one common portion 1190 for a set of multiple sense modules 1180. In
one example, a sense block will include one common portion 1190 and eight
sense modules 1180. Each of the sense modules in a group will communicate
with the associated common portion via a data bus 1172. For further
details refer to U.S. Patent Application Pub No. 2006/0140007, titled
"Non-Volatile Memory and Method with Shared Processing for an Aggregate
of Sense Amplifiers" published Jun. 29, 2006, and incorporated herein by
reference in its entirety.

[0121]Sense module 1180 comprises sense circuitry 1170 that determines
whether a conduction current in a connected bit line is above or below a
predetermined threshold level. Sense module 1180 also includes a bit line
latch 1182 that is used to set a voltage condition on the connected bit
line. For example, a predetermined state latched in bit line latch 1182
will result in the connected bit line being pulled to a state designating
program inhibit (e.g., VDD).

[0122]Common portion 1190 comprises a processor 1192, a set of data
latches 1194 and an I/O Interface 1196 coupled between the set of data
latches 1194 and data bus 1120. Processor 1192 performs computations. For
example, one of its functions is to determine the data stored in the
sensed storage element and store the determined data in the set of data
latches. The set of data latches 1194 is used to store data bits
determined by processor 1192 during a read operation. It is also used to
store data bits imported from the data bus 1120 during a program
operation. The imported data bits represent write data meant to be
programmed into the memory. I/O interface 1196 provides an interface
between data latches 1194 and the data bus 1120.

[0123]During read or sensing, the operation of the system is under the
control of state machine 1112 that controls the supply of different
control gate voltages to the addressed storage element. As it steps
through the various predefined control gate voltages corresponding to the
various memory states supported by the memory, the sense module 1180 may
trip at one of these voltages and an output will be provided from sense
module 1180 to processor 1192 via bus 1172. At that point, processor 1192
determines the resultant memory state by consideration of the tripping
event(s) of the sense module and the information about the applied
control gate voltage from the state machine via input lines 1193. It then
computes a binary encoding for the memory state and stores the resultant
data bits into data latches 1194. In another embodiment of the core
portion, bit line latch 1182 serves double duty, both as a latch for
latching the output of the sense module 1180 and also as a bit line latch
as described above.

[0124]Some implementations can include multiple processors 1192. In one
embodiment, each processor 1192 will include an output line (not
depicted) such that each of the output lines is wired-OR'd together. In
some embodiments, the output lines are inverted prior to being connected
to the wired-OR line. This configuration enables a quick determination
during the program verification process of when the programming process
has completed because the state machine receiving the wired-OR can
determine when all bits being programmed have reached the desired level.
For example, when each bit has reached its desired level, a logic zero
for that bit will be sent to the wired-OR line (or a data one is
inverted). When all bits output a data 0 (or a data one inverted), then
the state machine knows to terminate the programming process. Because
each processor communicates with eight sense modules, the state machine
needs to read the wired-OR line eight times, or logic is added to
processor 1192 to accumulate the results of the associated bit lines such
that the state machine need only read the wired-OR line one time.
Similarly, by choosing the logic levels correctly, the global state
machine can detect when the first bit changes its state and change the
algorithms accordingly.

[0125]During program or verify, the data to be programmed is stored in the
set of data latches 1194 from the data bus 1120. The program operation,
under the control of the state machine, comprises a series of programming
voltage pulses applied to the control gates of the addressed storage
elements. Each programming pulse is followed by a read back (verify) to
determine if the storage element has been programmed to the desired
memory state. Processor 1192 monitors the read back memory state relative
to the desired memory state. When the two are in agreement, the processor
1192 sets the bit line latch 1182 so as to cause the bit line to be
pulled to a state designating program inhibit. This inhibits the storage
element coupled to the bit line from further programming even if
programming pulses appear on its control gate. In other embodiments the
processor initially loads the bit line latch 1182 and the sense circuitry
sets it to an inhibit value during the verify process.

[0126]Data latch stack 1194 contains a stack of data latches corresponding
to the sense module. In one embodiment, there are three data latches per
sense module 1180. In some implementations (but not required), the data
latches are implemented as a shift register so that the parallel data
stored therein is converted to serial data for data bus 1120, and vice
versa. In the preferred embodiment, all the data latches corresponding to
the read/write block of m storage elements can be linked together to form
a block shift register so that a block of data can be input or output by
serial transfer. In particular, the bank of r read/write modules is
adapted so that each of its set of data latches will shift data in to or
out of the data bus in sequence as if they are part of a shift register
for the entire read/write block.

[0128]FIG. 14 illustrates an example of an organization of a memory array
into blocks for an all bit line memory architecture or for an odd-even
memory architecture. Exemplary structures of memory array 1400 are
described. As one example, a NAND flash EEPROM is described that is
partitioned into 1,024 blocks. The data stored in each block can be
simultaneously erased. In one embodiment, the block is the minimum unit
of storage elements that are simultaneously erased. In each block, in
this example, there are 8,512 columns corresponding to bit lines BL0,
BL1, . . . BL8511. In one embodiment referred to as an all bit line (ABL)
architecture (architecture 1410), all the bit lines of a block can be
simultaneously selected during read and program operations. Storage
elements along a common word line and connected to any bit line can be
programmed at the same time.

[0129]In the example provided, 64 storage elements and two dummy storage
elements are connected in series to form a NAND string. There are sixty
four data word lines and two dummy word lines, WL-d0 and WL-d1, where
each NAND string includes sixty four data storage elements and two dummy
storage elements. In other embodiments, the NAND strings can have more or
less than 64 data storage elements and two dummy storage elements. Data
memory cells can store user or system data. Dummy memory cells are
typically not used to store user or system data.

[0130]One terminal of the NAND string is connected to a corresponding bit
line via a drain select gate (connected to select gate drain lines SGD),
and another terminal is connected to c-source via a source select gate
(connected to select gate source line SGS).

[0131]In one embodiment, referred to as an odd-even architecture
(architecture 1400), the bit lines are divided into even bit lines (BLe)
and odd bit lines (BLo). In this case, storage elements along a common
word line and connected to the odd bit lines are programmed at one time,
while storage elements along a common word line and connected to even bit
lines are programmed at another time. Data can be programmed into
different blocks and read from different blocks concurrently. In each
block, in this example, there are 8,512 columns that are divided into
even columns and odd columns.

[0132]During one configuration of read and programming operations, 4,256
storage elements are simultaneously selected. The storage elements
selected have the same word line and the same kind of bit line (e.g.,
even or odd). Therefore, 532 bytes of data, which form a logical page,
can be read or programmed simultaneously, and one block of the memory can
store at least eight logical pages (four word lines, each with odd and
even pages). For multi-state storage elements, when each storage element
stores two bits of data, where each of these two bits are stored in a
different page, one block stores sixteen logical pages. Other sized
blocks and pages can also be used.

[0133]For either the ABL or the odd-even architecture, storage elements
can be erased by raising the p-well to an erase voltage (e.g., 20 V) and
grounding the word lines of a selected block. The source and bit lines
are floating. Erasing can be performed on the entire memory array,
separate blocks, or another unit of the storage elements which is a
portion of the memory device. Electrons are transferred from the floating
gates of the storage elements to the p-well region so that the VTH
of the storage elements becomes negative.

[0134]FIG. 15 depicts an example set of threshold voltage distributions.
Example VTH distributions for the storage element array are provided
for a case where each storage element stores two bits of data. A first
threshold voltage distribution E is provided for erased storage elements.
Three threshold voltage distributions, A, B and C for programmed storage
elements, are also depicted. In one embodiment, the threshold voltages in
the E distribution are negative and the threshold voltages in the A, B
and C distributions are positive.

[0135]Each distinct threshold voltage range corresponds to predetermined
values for the set of data bits. The specific relationship between the
data programmed into the storage element and the threshold voltage levels
of the storage element depends upon the data encoding scheme adopted for
the storage elements. For example, U.S. Pat. No. 6,222,762 and U.S.
Patent Application Publication No. 2004/0255090, published Dec. 16, 2004,
both of which are incorporated herein by reference in their entirety,
describe various data encoding schemes for multi-state flash storage
elements. In one embodiment, data values are assigned to the threshold
voltage ranges using a Gray code assignment so that if the threshold
voltage of a floating gate erroneously shifts to its neighboring physical
state, only one bit will be affected. One example assigns "11" to
threshold voltage range E (state E), "10" to threshold voltage range A
(state A), "00" to threshold voltage range B (state B) and "01" to
threshold voltage range C (state C). However, in other embodiments, Gray
code is not used. Although four states are shown, the present invention
can also be used with other multi-state structures including those that
include more or fewer than four states.

[0136]Three read reference voltages, Vra, Vrb and Vrc, are also provided
for reading data from storage elements. By testing whether the threshold
voltage of a given storage element is above or below Vra, Vrb and Vrc,
the system can determine the state, e.g., programming condition, the
storage element is in.

[0137]Further, three verify reference voltages, Vva, Vvb and Vvc, are
provided. Additional read and reference values can be used when the
storage elements store additional states. When programming storage
elements to state A, the system will test whether those storage elements
have a threshold voltage greater than or equal to Vva. When programming
storage elements to state B, the system will test whether the storage
elements have threshold voltages greater than or equal to Vvb. When
programming storage elements to state C, the system will determine
whether storage elements have their threshold voltage greater than or
equal to Vvc.

[0138]In one embodiment, known as full sequence programming, storage
elements can be programmed from the erase state E directly to any of the
programmed states A, B or C. For example, a population of storage
elements to be programmed may first be erased so that all storage
elements in the population are in erased state E. A series of programming
pulses such as depicted by the control gate voltage sequence of FIG. 19
will then be used to program storage elements directly into states A, B
or C. While some storage elements are being programmed from state E to
state A, other storage elements are being programmed from state E to
state B and/or from state E to state C. When programming from state E to
state C on a selected word line, WLi, the amount of parasitic coupling to
the adjacent floating gate under WLi-1 is a maximized since the change in
amount of charge on the floating gate under WLi is largest as compared to
the change in voltage when programming from state E to state A or state E
to state B. When programming from state E to state B the amount of
coupling to the adjacent floating gate is reduced but still significant.
When programming from state E to state A the amount of coupling is
reduced even further. Consequently the amount of correction required to
subsequently read each state of WLi-1 will vary depending on the state of
the adjacent storage element on WLi.

[0139]FIG. 16 illustrates an example of a two-pass technique of
programming a multi-state storage element that stores data for two
different pages: a lower page and an upper page. Four states are
depicted: state E (11), state A (10), state B (00) and state C (01). For
state E, both pages store a "1." For state A, the lower page stores a "0"
and the upper page stores a "1." For state B, both pages store "0." For
state C, the lower page stores "1" and the upper page stores "0." Note
that although specific bit patterns have been assigned to each of the
states, different bit patterns may also be assigned.

[0140]In a first programming pass, the storage element's threshold voltage
level is set according to the bit to be programmed into the lower logical
page. If that bit is a logic "1," the threshold voltage is not changed
since it is in the appropriate state as a result of having been earlier
erased. However, if the bit to be programmed is a logic "0," the
threshold level of the storage element is increased to be state A, as
shown by arrow 1600. That concludes the first programming pass.

[0141]In a second programming pass, the storage element's threshold
voltage level is set according to the bit being programmed into the upper
logical page. If the upper logical page bit is to store a logic "1," then
no programming occurs since the storage element is in one of the states E
or A, depending upon the programming of the lower page bit, both of which
carry an upper page bit of "1." If the upper page bit is to be a logic
"0," then the threshold voltage is shifted. If the first pass resulted in
the storage element remaining in the erased state E, then in the second
phase the storage element is programmed so that the threshold voltage is
increased to be within state C, as depicted by arrow 1620. If the storage
element had been programmed into state A as a result of the first
programming pass, then the storage element is further programmed in the
second pass so that the threshold voltage is increased to be within state
B, as depicted by arrow 1610. The result of the second pass is to program
the storage element into the state designated to store a logic "0" for
the upper page without changing the data for the lower page. In both FIG.
15 and FIG. 16, the amount of coupling to the floating gate on the
adjacent word line depends on the final state.

[0142]In one embodiment, a system can be set up to perform full sequence
writing if enough data is written to fill up an entire page. If not
enough data is written for a full page, then the programming process can
program the lower page programming with the data received. When
subsequent data is received, the system will then program the upper page.
In yet another embodiment, the system can start writing in the mode that
programs the lower page and convert to full sequence programming mode if
enough data is subsequently received to fill up an entire (or most of a)
word line's storage elements. More details of such an embodiment are
disclosed in U.S. Patent Application Pub. No. 2006/0126390, titled
"Pipelined Programming of Non-Volatile Memories Using Early Data,"
published Jun. 15, 2006, incorporated herein by reference in its
entirety.

[0143]FIGS. 17a-c disclose another process for programming non-volatile
memory that reduces the effect of floating gate to floating gate coupling
by, for any particular storage element, writing to that particular
storage element with respect to a particular page subsequent to writing
to adjacent storage elements for previous pages. In one example
implementation, the non-volatile storage elements store two bits of data
per storage element, using four data states. For example, assume that
state E is the erased state and states A, B and C are the programmed
states. State E stores data 11. State A stores data 01. State B stores
data 10. State C stores data 00. This is an example of non-Gray coding
because both bits change between adjacent states A and B. Other encodings
of data to physical data states can also be used. Each storage element
stores two pages of data. For reference purposes, these pages of data
will be called upper page and lower page; however, they can be given
other labels. With reference to state A, the upper page stores bit 0 and
the lower page stores bit 1. With reference to state B, the upper page
stores bit 1 and the lower page stores bit 0. With reference to state C,
both pages store bit data 0.

[0144]The programming process is a two-step process. In the first step,
the lower page is programmed. If the lower page is to remain data 1, then
the storage element state remains at state E. If the data is to be
programmed to 0, then the threshold of voltage of the storage element is
raised such that the storage element is programmed to state B'. FIG. 17a
therefore shows the programming of storage elements from state E to state
B'. State B' is an interim state B; therefore, the verify point is
depicted as Vvb', which is lower than Vvb.

[0145]In one embodiment, after a storage element is programmed from state
E to state B', its neighbor storage element (WLn+1) in the NAND string
will then be programmed with respect to its lower page. For example,
looking back at FIG. 2, after the lower page for storage element 106 is
programmed, the lower page for storage element 104 would be programmed.
After programming storage element 104, the floating gate to floating gate
coupling effect will raise the apparent threshold voltage of storage
element 106 if storage element 104 had a threshold voltage raised from
state E to state B'. This will have the effect of widening the threshold
voltage distribution for state B' to that depicted as threshold voltage
distribution 1750 of FIG. 17b. This apparent widening of the threshold
voltage distribution will be remedied when programming the upper page.

[0146]FIG. 17c depicts the process of programming the upper page. If the
storage element is in erased state E and the upper page is to remain at
1, then the storage element will remain in state E. If the storage
element is in state E and its upper page data is to be programmed to 0,
then the threshold voltage of the storage element will be raised so that
the storage element is in state A. If the storage element was in
intermediate threshold voltage distribution 1750 and the upper page data
is to remain at 1, then the storage element will be programmed to final
state B. If the storage element is in intermediate threshold voltage
distribution 1750 and the upper page data is to become data 0, then the
threshold voltage of the storage element will be raised so that the
storage element is in state C. The process depicted by FIGS. 17a-c
reduces the effect of floating gate to floating gate coupling because
only the upper page programming of neighbor storage elements will have an
effect on the apparent threshold voltage of a given storage element. An
example of an alternate state coding is to move from distribution 1750 to
state C when the upper page data is a 1, and to move to state B when the
upper page data is a 0.

[0147]Although FIGS. 17a-c provide an example with respect to four data
states and two pages of data, the concepts taught can be applied to other
implementations with more or fewer than four states and different than
two pages. For example, FIGS. 5a-d discussed an embodiment with three
pages: lower, middle and upper.

[0148]FIG. 18 is a flow chart describing one embodiment of a method for
programming non-volatile memory. In one implementation, storage elements
are erased (in blocks or other units) prior to programming. In step 1800,
a "data load" command is issued by the controller and input received by
control circuitry 1110. In step 1805, address data designating the page
address is input to decoder 1114 from the controller or host. In step
1810, a page of program data for the addressed page is input to a data
buffer for programming. That data is latched in the appropriate set of
latches. In step 1815, a "program" command is issued by the controller to
state machine 1112.

[0149]Triggered by the "program" command, the data latched in step 1810
will be programmed into the selected storage elements controlled by state
machine 1112 using the stepped program pulses of the pulse train 1900 of
FIG. 19 applied to the appropriate selected word line. In step 1820, the
program voltage, VPGM, is initialized to the starting pulse (e.g.,
12 V or other value) and a program counter (PC) maintained by state
machine 1112 is initialized at zero. In step 1830, the first VPGM
pulse is applied to the selected word line to begin programming storage
elements associated with the selected word line. If logic "0" is stored
in a particular data latch indicating that the corresponding storage
element should be programmed, then the corresponding bit line is
grounded. On the other hand, if logic "1" is stored in the particular
latch indicating that the corresponding storage element should remain in
its current data state, then the corresponding bit line is connected to
Vdd to inhibit programming.

[0150]In step 1835, the states of the selected storage elements are
verified. If it is detected that the target threshold voltage of a
selected storage element has reached the appropriate level, then the data
stored in the corresponding data latch is changed to a logic "1." If it
is detected that the threshold voltage has not reached the appropriate
level, the data stored in the corresponding data latch is not changed. In
this manner, a bit line having a logic "1" stored in its corresponding
data latch does not need to be programmed. When all of the data latches
are storing logic "1," the state machine (via the wired-OR type mechanism
described above) knows that all selected storage elements have been
programmed. In step 1840, a check is made as to whether all of the data
latches are storing logic "1." If all of the data latches are storing
logic "1," the programming process is complete and successful because all
selected storage elements were programmed and verified. A status of
"PASS" is reported in step 1845.

[0151]If, in step 1840, it is determined that not all of the data latches
are storing logic "1," then the programming process continues. In step
1850, the program counter PC is checked against a program limit value
PCmax. One example of a program limit value is twenty; however, other
numbers can also be used. If the program counter PC is not less than
PCmax, then the program process has failed and a status of "FAIL" is
reported in step 1855. If the program counter PC is less than PCmax, then
VPGM is increased by the step size and the program counter PC is
incremented in step 1860. The process then loops back to step 1830 to
apply the next VPGM pulse.

[0152]FIG. 19 depicts an example pulse train 1900 applied to the control
gates of non-volatile storage elements during programming, and a switch
in boost mode which occurs during a pulse train. The pulse train 1900
includes a series of program pulses 1905, 1910, 1915, 1920, 1925, 1930,
1935, 1940, 1945, 1950, . . . , that are applied to a word line selected
for programming. In one embodiment, the programming pulses have a
voltage, VPGM, which starts at 12 V and increases by increments,
e.g., 0.5 V, for each successive programming pulse until a maximum of 20
V is reached. In between the program pulses are verify pulses. For
example, verify pulse set 1906 includes three verify pulses. In some
embodiments, there can be a verify pulse for each state that data is
being programmed into, e.g., state A, B and C. In other embodiments,
there can be more or fewer verify pulses. The verify pulses in each set
can have amplitudes of Vva, Vvb and Vvc (FIG. 16) or Vvb' (FIG. 17a), for
instance.

[0153]As mentioned, the voltages which are applied to word lines to
implement a boost mode are applied when programming occurs, e.g., prior
to and during a program pulse. In practice, the boost voltages of a boost
mode can be initiated slightly before each program pulse and removed
after each program pulse. On the other hand, during the verify process,
for instance, which occurs between program pulses, the boost voltages are
not applied. Instead, read voltages, which are typically less than the
boost voltages, are applied to the unselected word lines. The read
voltages have an amplitude which is sufficient to maintain the previously
programmed storage elements in a NAND string on when the threshold
voltage of a currently-programmed storage element is being compared to a
verify level.

[0154]The foregoing detailed description of the invention has been
presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise form
disclosed. Many modifications and variations are possible in light of the
above teaching. The described embodiments were chosen in order to best
explain the principles of the invention and its practical application, to
thereby enable others skilled in the art to best utilize the invention in
various embodiments and with various modifications as are suited to the
particular use contemplated. It is intended that the scope of the
invention be defined by the claims appended hereto.